To cite package 'piano' in publications use:

  Väremo, L., Nielsen, J. and Nookaew, I. (2013) Enriching the gene set
  analysis of genome-wide data by incorporating directionality of gene
  expression and combining statistical hypotheses and methods. Nucleic
  Acids Research. 41 (8), 4378-4391. doi: 10.1093/nar/gkt111

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Enriching the gene set analysis of genome-wide data by incorporating directionality of gene expression and combining statistical hypotheses and methods},
    author = {Leif V\"{a}remo and Jens Nielsen and Intawat Nookaew},
    year = {2013},
    journal = {Nucleic Acids Research},
    volume = {41},
    number = {8},
    pages = {4378-4391},
    doi = {10.1093/nar/gkt111},
  }